Dog
Stranging Vine (Cynanchum
rossicum,
formally of the family Vincetoxicum)is a European member of the milkweed family also known as Pale
(or European) Swallowort. Dog Strangling Vine (DSV) has become
invasive in many
parts of eastern North America and is particularly abundant in the GTA
and parts of Ottawa. It is commonly found along roadsides and
railways, as the seeds are picked up by passing vehicles and railcars
and distributed throughout the province. Both DSV and its close
relative Black
Swallowort (Cynanchum nigrum,
also known as Cynanchum louisaea
and sometimes referred to as dog strangling vine)
form dense colonies in a variety of habitats, where they outcompete
other vegetation and literally strangle plants with their twining
growth habit.

Leaves of
DSV are opposite and short
stalked, oblong to oval with pointed tips, 5-10 cm long and dark green
(Dickinson et al, 2004),
although it has been our observation that the leaves are lighter and
more pointed when the plant is growing in full sun than when it grows
in shade. The pale swallowort flowers are longer than they are
broad and pink to light purple in colour, whereas the black swallowort
flowers are more triangular in shape (with microscopic hairs) and dark
purple with a yellow center. Pale
swallowort spreads to new areas primarily by seed, whereas black
swallowort forms deep rhizomes which aid its spread (data from the USDA
Natural
Resources Conservation Services website, see below).

In addition
to its disruptive growth
pattern, DSV carries substances that
repel or kill native herbivorous insects. These plants are
particularly
problematic for the Monarch Butterfly populations as the females lay
their eggs on the DSV (due to their close relation to milkweeds) but
the larvae cannot mature on these plants due to their inherent
toxicity. There are several other species of insects that also
oviposit on milkweed that may also be affected by this invasive
species. These plants are not considered weeds in their native
European habitats as there are a variety of insects that are
herbivorous on the leaves and/or seeds, suggesting that biocontrol may
be a suitable means of control or that native species may be found that
can tolerate and inhibit DSV's invasive growth.

Control methods:

Pulling
Pulling or digging out small DSV plants by their roots is an effective
measure in the early years of establishment. You must be diligent
because if only the above ground portion of the plants are removed
these plants will readily re-sprout from the remaining root system.
This measure should be applied when the soil is moist and the roots are
easily loosened to avoid soil disturbance. Soil disturbance may also
activate the seed bank.

Limiting dispersal of seeds
As the DSV seeds disperse by wind, gardeners and home owners must be
vigilant and survey land especially in the fruiting season (August,
September and October). This plant turns bright yellow and pods are
easily visible during late summer. Remove the pods, place in bags and
discard in a green bin. Do not put these into
compost. The fall is the best time for above ground removal of
the plant colony to stop seed dispersal.

Mulching
Where DSV growth is dense, mulching with multiple layers of newspapers,
leaf or twigs and cloths are fairly effective when used around the base
of trees and shrubs. Although some plants grow through the mulch,
growth is considerably less than in un-mulched areas nearby (see page
regarding Field Trials). Mulching
with
bio-degradable plastic is also very effective if the plastic is covered
with wood chips. Contact us
for information on obtaining biodegradable plastic.

Chemical control
Repeated application of the herbicides glyphosate (Roundup) and
triclopyr (Brush-B-Gon or Garlon 3A) are both effective as foliar
sprays during the seasons of larger infestation. The first
application should be done at the onset of flowering. There
should be a follow-up treatment in two to three weeks. For dense
populations additional treatments may be required for the next couple
of years. Always be aware of the potential risks in the use of
pesticides because of their toxicity and impacts on non-target species.

Biological control
A number of species of insects feed onwhite swallowort (C. hirundinaria) in Europe
including the weevil Otiorhynchus
pinastri, the diptera Euphranta
connexa, the gall midges Contarinia
vincetoxici and C. asclepiadis,
and several species of leaf beetles including Chrysochus
asclepiadeus.
Herbivory of these insects on black or pale swallowort has generally
not been confirmed. There are currently studies underway through
CABI investigating a selection of insects as potential biocontrol
agents.
Only a few native species have been reported to feed on dog strangling
vine, including milkweed longhorn beetle (Tetraopes tetraopthalmus),
small milkweed bug (Lygæus kalmii), large milkweed bug (cf. Oncopeltus
fasciatus) and Oleander aphids (Aphis nerii), although there has been
no indication that the insects thrive on this plant or that they cause
any significant damage (TRCA, 2007). Shotgun damage on DSV leaves
has been found to be caused by two separate species to date - by the
colorado potato beetle (Leptinotarsa decemlineata) (TRCA, 2007) and
also by the tarnished plant bug (Lygus lineolaris) (USDA NRCS website),
but this damage also has not been shown to have a significant impact on
plant growth. Our own studies also identified an additional
insect creating this type of damage (see herbivory).

Plants with vigourous early season growth caste dense shade and can
help
in controlling DSV. Dense tree cover can also act as a barrier for seed
production and dispersal. Tree planting could be an effective method to
block the major corridors.

Planting of trees and shrubs that inhibit the growth of DSV by
releasing chemicals (allelopathy) is another option for biological
control.
Black walnut (Juglans nigra) is well-known for its allelopathic effects
(secretion of Juglone). Our own observations and those of other
researchers (TRCA, 2007) have described instances where black walnut
did appear to inhibit DSV within its drip-line.

Please email us directly if you
have seen any instances of insect damage on DSV in your area, and
include any information you may be able to provide regarding the type
of damage, extent of the damage, and possible insect candidates.
If you have confirmed herbivory by a particular insect, please let us
know!